WASHINGTON – The Obama administration has finalized new fuel economy rules that will force automakers to nearly double the average gas mileage of all new cars and trucks sold by 2025.

The rules mean that the average miles per gallon must hit 54.5 in 13 years, up from 28.6 mpg at the end of last year. The requirements will be phased in gradually between now and then, and automakers could be fined if they don’t comply.

The regulations, announced Tuesday, will change the cars and trucks sold in U.S. showrooms, with the goal of slashing greenhouse gas emissions and fuel consumption. Automakers will need to improve gasoline-powered engines and sell more alternative fuel vehicles.

Critics say the rules will make cars unaffordable by adding thousands of dollars to the sticker price.

The Corporate Average Fuel Economy standards will vary by automaker depending on the mix of models they sell. The requirements will be lower for companies such as General Motors, Ford and Chrysler, which offer more pickup trucks.

The standards can be lowered by the government if people suddenly start buying less-efficient vehicles in the future, although few expect that to happen.

The administration says the latest changes will save families up to $7,400 on fuel over the life of a vehicle. The standards also are the biggest step the U.S. government has ever taken toward cutting greenhouse gas emissions, Environmental Protection Agency Administrator Lisa Jackson said. Tailpipe emissions from cars and light trucks will be halved by 2025.

President Obama said the new fuel standards “represent the single most important step” his administration has taken to reduce U.S. dependence on foreign oil.

But Republican presidential nominee Mitt Romney has opposed the standards, and his campaign Tuesday said any savings at the pump would be wiped out by the rising cost of cars and trucks.

Already, automakers have committed to an average of 35.5 mpg by model year 2016 under a deal reached with the Obama administration three years ago.

In the arcane world of government regulations, the rules don’t mean that each new car or truck will get 54.5 mpg. The average vehicle will get closer to 40 mpg in real-world driving. Automakers will be able to sell pickup trucks and less-efficient vehicles as long as that’s offset somewhat by smaller vehicles that already can get upward of 40 mpg.

Automakers can reduce the mileage they’re required to get with credits for selling natural gas and electric vehicles, changing air conditioning fluid to one that pollutes less, and adding stop-start circuits.